CRT displays are widely used as display devices for television receivers, video monitors and apparatuses such as word processors, computers, simulators, synchroscopes, instruments for analysis, medical machines, gaming machines, and the like, and among them, in particular, word processors, computers, are machines which are at present found generally in offices.
However, with the spread of apparatuses such as work processors, computers, and the like, the number of operators who closely observe CRT displays for long period of time also continually increases. In advanced nations, the occupational diseases of operators who operate these apparatuses is becoming a considerable social problem.
Fatigue of the optic nerve is a symptom which is common to these occupational diseases. Amongst factors which exert an adverse influences, it is first considered that the light in the room and sunlight reflected by the CRT screen and entering the eyes of the operators cause fatigue of the optic nerve. In addition, although the sequence of cause and effect has not been sufficiently analysed yet by scientific methods, it is also necessary to consider the influences of electromagnetic waves, electrostatic charge, ultraviolet rays, and the like which are generated by the CRT.
To solve the foregoing problems which result from CRT displays, there has been developed a method for reducing the glare of the light by making the screen surface of the CRT rough, a flat filter of a low reflection factor and a filter having a conductive layer, attached to the front of the CRT screen, in order to reduce the glare of the light, and so on.
Techniques to make the surface rough to reduce the glare of the light of the CRT display have been disclosed in, for example, Japanese Patent Laid-open Publications Nos. 55-12107, 56-32653 and 60-125681.
On the other hand, techniques to directly use a net as a filter have been disclosed in, for example, Japanese Utility Model Laid-open Publication No. 60-107802, Japanese Patent Laid-Open Publications Nos. 57-126054 and 61-46903. Further, a detachable filter of the type which is closely adhered onto the surface of the CRT has been set forth in, for example, U.S. Pat. No. 4,253,737 and Japanese Utility Model Laid-open Publication No. 59-106176.
Again, a conductive filter for a CRT screen has been shown in, for example, Japanese Utility Model Laid-open Publication Nos. 59-106176 and 60-125681, and Patent Laid-open Publication No. 61-15480, Such a filter is a one having a thin metal layer which is formed by evaporation or sputtering onto the surface of the filter, or else embodying a net consisting of conductive fiber. Further, there has been proposed a method whereby a transparent resin filter containing lead or the like is attached to the CRT screen, thereby minimizing harmful influences of the electromagnetic waves, static electricity, and the like.
However, the CRTs which are generally used have the curved screens, so that is a flat filter is attached to the screen, the display image at the corners of the screen is distorted and becomes hard to see.
Therefore, the operator who works with a CRT prefers not to attach a filter, with the disadvantage that working with the filter detached frequently occurs. In addition, when a flat filter is attached to the curved screen of a CRT, the electromagnetic waves can leak from the gap between the CRT screen adn the filter, so that the screening effect is reduced.
Since the existing flat filters need special frames adapted to be attached to the CRT, there is the disadvantage of a significant increase in cost.
In the case of the filter comprising a deposited metal layer on the surface, distortion at the corners of the CRT screen is not caused; however, it has the disadvantages that glare-proofing is difficult and fingerprint marking and the like are conspicuous.
On the other hand, in the case of a CRT filter consisting of a net, the glare of the light can be remarkably reduced; however, upon attaching this filter closely to the CRT screen, there rises a problem that moire fringes can be produced between the scanning lines of the CRT and lines of dots of the shadow mask or the wires of a net. In particular, when a filter of the net type is closely attached to a color CRT, even if the meshes or wires of the net are disposed of at an angle to the scanning lines, the moire fringes cannot be prevented by this means.
Further, when a filter having a metal layer deposited by evaporation onto the outer surface, or a filter on which is laminated a net consisting of conductive fiber is attached to the CRT, and if a hand or the like touches the filter surface, the operator often receives an electric shock due to electrostatic charge, and further, in the case of cleaning the surface of such a filter, there are problems in that the metal layer of the surface or the net consisting of conductive fiber may be abraded or peeled off, so that the conductivity and glare-proofing are lost.